1. Field of the Invention
The present invention relates to a hydroentangling process and, more particularly, to particular configurations of an orifice-type jet strip device used in a hydroentangling process.
2. Description of Related Art
Hydroentanglement or “spunlacing” is a process used for mechanically bonding a web of loose fibers to directly form a fabric. Such a class of fabric belongs to the “nonwoven” family of engineered fabrics. The underlying mechanism in hydroentanglement is the subjecting the fibers to a non-uniform pressure field created by a successive bank of high-velocity waterjets. The impact of the waterjets with the fibers, while the fibers are in contact with adjacent fibers, displaces and rotates the adjacent fibers, thereby causing entanglement of the fibers. During these relative displacements of the fibers, some of the fibers twist around others and/or inter-lock with other fibers to form a strong structure, due at least in part, to frictional forces between the interacting fibers. The resulting product is a highly compressed and uniform fabric formed from the entangled fibers. Such a hydroentangled fabric is often highly flexible, yet very strong, generally outperforming woven and knitted fabric counterparts in performance. The hydroentanglement process is thus a high-speed low-cost alternative to other methods of producing fabrics. Hydroentanglement machines can, for example, run (produce the fabric) as fast as about 700 meters of fabric or more per minute, wherein the fabric may be, for instance, between about 1 and about 6 meters wide. In operation, the hydroentanglement process depends on particular properties of coherent high-speed waterjets produced by directing pressurized water through special nozzles.
Axially-extending hydroentangling nozzles are traditionally made up of two sections or portions. A cylindrical section (capillary portion) typically comprises the fluid inlet to the nozzle and having a diameter, for example, of about 120 microns. The capillary portion is fluidly connected to a cone portion having, for instance, a cone angle of about 15 degrees, though the cone angle may vary considerably. In practice, hydroentangling waterjets are emitted through one or more relatively thin plate strips on the order of between about 1 meter and about 6 meters long, and having between about 1600 and about 2000 orifices or nozzles per meter (see, e.g., FIG. 1). Manufacturing thousands of such small orifices or nozzles in close proximity to each other results in many constraints on the design process for the device. Typically, a jet strip is in the form of a thin-plate strip having a thickness, for example, of about 1 millimeter. Such manufacturing limitations are in part, responsible for the cone-capillary geometry that has generally been used since the inception of hydroentangling process. While this jet strip geometry has worked well in the past thirty years, changes in process parameters have resulted in a need for an improved and more durable jet strip. For example, the operating pressures employed in the hydroentangling process for forcing the fluid through the orifices or nozzles in the plate strip have increased from about 100 bars to over 500 bars. Due to the forces, imparted to the jet strip by the increased pressure of the pressurized fluid, the jet strip (nozzles) tends to wear on an accelerated basis. Additionally, such higher fluid pressures may also lead to a different profile of the waterjet for the same nozzle geometry. Accordingly, process and conditions that worked well for nozzles at low fluid pressures need to be modified for high-pressure waterjets produced through the nozzles, thereby indicating that existing orifice (nozzle) geometries or other configurations are not optimal for high-pressure waterjets.
The geometry of the orifice (also referred to herein as “nozzle” or “nozzle orifice”) generally has a significant impact on the coherence of the discharged waterjets (see, e.g., Lin S. P., Reitz R. D. (1998), Drop and spray formation from a liquid jet, Ann. Rev. Fluid Mech., Vol. 30; Wu P.-K., Miranda R. F. and Faeth G. M. (1995) Effects of initial flow conditions on primary breakup of non-turbulent and turbulent round liquid jets, Atomization and sprays, Vol. 5, pp. 175-196; or Vahedi Tafreshi H. and B. Pourdeyhimi (2003) “Effects of Nozzle Geometry on Waterjet Breakup at High Reynolds Numbers”, Experiments in Fluids, (35) 364-371). In the case of a sharp-edge waterjet orifice, a jet strip in the form of a plate separates a pressurized body of water (in a manifold or other suitable device) from the downstream air (the hydroentanglement process area), and the nozzles extend through the major surfaces of the plate, from the pressurized body of water to the downstream air, with a sharp transition between the major surface of the plate facing the body of water and the respective nozzle. The pressurized water thus enters the nozzle in a water flow, wherein the sharp edge causes the flow to detach from the nozzle wall at the fluid inlet (capillary portion) of the nozzle and form a vena contracta (necked configuration) upon entry into the capillary portion. Depending on the length of the capillary portion and the hydrodynamics or other parameters of the water flow, the water flow may or may not reattach to the wall after some distance (see, e.g., Lefebvre A. H. (1989) Atomization and Sprays” Hemisphere Publishing Corporation; or Bayvel, L., and Orzechowski Z. (1993) Liquid Atomization, Taylor & Francis).
Detached flows have certain characteristics that make such flows beneficial in some applications. In the case of detached flows, there is an air gap between the liquid and the capillary wall, generally following the fluid entrance or inlet into the capillary. This air may tend to envelop the liquid flow all the way through the capillary and thus may not allow any contact between liquid phase flow and the capillary wall. Accordingly, in such an instance, wall-induced friction and cavitation do not disturb the structure of this flow. A waterjet resulting from such a detached flow, also termed a constricted waterjet, has a higher stability and therefore, a longer breakup length (see, e.g., Hiroyasu H. (2000), Spray Breakup Mechanism from the Hole-type Nozzle and Its Applications, Atomization and Sprays, Vol. 10, pp. 511-521; or Vahedi Tafreshi and Pourdeyhimi 2003). The constricted waterjets may stay laminar even at relatively high Reynolds numbers, as opposed to non-constricted waterjets. FIG. 2 shows a graphical comparison between constricted and non-constricted waterjets issued at the same Reynolds number.
A constricted jet is formed when the water flow enters the capillary portion of a cone-capillary type nozzle shown, for example, in FIG. 1. A non-constricted jet is formed when water enters such a nozzle from the conical side. Such configurations are herein referred to as cone-down and cone-up type nozzles, respectively. The apparently unbroken portion of the constricted waterjet shown, for example, in FIG. 2a is not actually a continuous jet of water. Such a statement is evidenced in FIG. 3 where the image of FIG. 2 is juxtaposed with high-speed images taken at three different locations along the waterjet. As shown in FIG. 3, the constricted waterjet includes a continuous region (FIG. 3b), a discrete region (FIG. 3c), and a spray region (FIGS. 3d and 3e). In the discrete region, the waterjet is primarily broken (i.e., broken into large droplets). Following the discrete region, large droplets appear, possibly as a secondary breakup resulting from the primary breakup, and the result is a spray of very fine droplets. Such fine droplets are shown in the pictures of the waterjet in FIGS. 3d and 3e. FIG. 3d illustrates the “bag breakup” or secondary breakup of the large drops resulting from the primary breakup.
Generally, the discharge coefficient of a nozzle, defined as the ratio of the real (experimental) flow rate from a nozzle to the flow rate calculated by using the inviscid one-dimensional flow theory (Bernoulli equation), is about 0.62 and 0.92, depending on whether the flow is detached or not, respectively (see, e.g., Ohm, T. R., Senser, D. W., and Lefebvre, H. (1991) “Geometrical effects on discharge coefficients for plain-orifice atomizers”, Atomization and Sprays, 1, pp. 137-153). With this in mind, A Computational Fluid Dynamics (CFD) code from Fluent Inc. was used to solve the unsteady state Reynolds-Averaged Navier-Stokes equations (RANS) in an axi-symmetric geometry. It was observed that, when water starts flowing into the capillary, initially filled with air, the water becomes detached from the capillary wall since the water, prior to the capillary inlet, gains momentum along the surface of the nozzle plate contacting the water source. The momentum of the water does not allow the water flow to perfectly follow the sudden 90-degree turn transition between the plate surface and the capillary wall. In this regard, FIG. 4 shows the frontline of a waterjet after entering a capillary portion of a nozzle, over a time sequence, for a Reynolds number of Re=21250, with detachment of the water flow from the capillary wall. More particularly, after about 1.2 microseconds, the frontline of the water jet enters the conical portion of the nozzle, but the water flow also reattaches to the capillary wall before completely progressing into the cone portion. Once the water flow reattaches to the nozzle wall, a re-circulating ring of air becomes entrapped inside the nozzle, between the detachment and reattachment points of the water jet. The air bubble will subsequently break up and the re-circulating air zone will become filled by water. The breakup of the air ring and dispersion thereof into the liquid phase, as shown in the latter stages of FIG. 4, causes a relatively large amount of disturbance and turbulence, which perturbs the integrity and collimation of the forming waterjet. Accordingly, once the reattachment of the water flow to the nozzle wall occurs, the waterjet will no longer be laminar and glassy through the nozzle.
The reattachment-induced breakup occurrence in a cone-capillary type nozzle, however, is typically not expected to occur in a conical type nozzle, as shown in FIG. 5a. The water flow progression shown in FIG. 5a is representative of a conical type nozzle having an inlet diameter of about 128 microns and 15-degree cone angle, operating with a Reynolds number of Re=21250. The air circulation inside the conical type nozzle is represented by the velocity vectors in FIG. 5b, after 1.6 microseconds of operation. The formed air gap thus envelops the waterjet and protects the water flow from nozzle wall-induced turbulence (see, e.g., Vahedi Tafreshi H. and B. Pourdeyhimi (2003) “Effects of Nozzle Geometry on Waterjet Breakup at High Reynolds Numbers”, Experiments in Fluids, (35) 364-371).
A reduction in the pressure of the water flow generally occurs in the separated (detached), but liquid-filled, region formed after the water flow enters the sharp-edged nozzle. If, however, the water flow velocity is high enough to cause the pressure on the separated or detached region to drop down to the water vapor pressure, vaporization will occur and a cavitation pocket will form (see, e.g., Knapp R. T., Daily J. W., and Hammitt F. G (1970) Cavitation, McGraw-Hill Inc.). Such cavitation disturbs the flow pattern within the nozzle (see, e.g., Schmidt D. P., Rutland C. J., Corradini M. L., Roosen P., and Genge O. (1999), Cavitation in Two Dimensional Asymmetric Nozzles, SAE Technical Series 1999-01-0518; Badock C., Wirth R., Fath A., Leipertz A. (1999), “Investigation of cavitation in real size diesel injection nozzles” International Journal of Heat and Fluid Flow, 20, 538-544; or Chaves, H., Knapp, M., Kubitzek, A., Obermeier, F., and Schneider T. (1995), Experimental Study of cavitation in the Nozzle Hole of Diesel Injectors Using Transparent Nozzles, SAE Papers, 1995-0290). With respect to the configuration shown in FIG. 4, when the water flow reattaches to the nozzle wall and the air ring becomes filled with water, cavitation starts in the initially air-filled recirculation zone. Cavitation bubbles can significantly disturb the steadiness of the nozzle water flow, and causes turbulence that accelerates the disintegration of the waterjet. If the rate of cavitation is so intense that cavitation cloud grows and reaches the nozzle outlet, the downstream air will flow up to the nozzle (against the water flow) and fill the low-pressure vapor/liquid filled re-circulation region (see, e.g., FIG. 6; Vahedi Tafreshi H. and Pourdeyhimi B. (2004a), Simulation of Cavitation and Hydraulic Flip inside Hydroentangling Nozzles, Textile Research Journal 74(4) 359-364; or Vahedi Tafreshi H. and Pourdeyhimi B. (2004b), Cavitation and Hydraulic Flip, FLUENT News, 13(1) 38). Once the reverse air flow occurs, the water flow will no longer be in contact with the capillary wall in the re-circulation zone. Therefore, cavitation ceases, a stable undisturbed stream of water flows through the nozzle, and a constricted waterjet forms. This phenomenon is otherwise referred to as “hydraulic-flip.”
Generally, over a relatively long time (“steady state”), there is little or no difference between a waterjet formed by hydraulic flip and a waterjet formed in perfectly cavitation-free process (e.g., as shown in FIG. 5a). As such, if the nozzle causes cavitation (FIG. 4) for the first few microseconds (or maybe milliseconds if the operating Reynolds number is less than 21250) of operation, the waterjet will not be collimated. Therefore, in applications where a collimated jet is required, even at very beginning of jet ejection (e.g., in inkjets printers), a determination of whether or not reattachment occurs inside the nozzle may be very important. In addition, besides affecting the waterjet integrity, cavitation can erode metallic surfaces (if the nozzle is made from a metallic material) and therefore, damage the nozzle shape. The collapse of the cavitation bubbles close to the nozzle wall surface generates a strong pressure wave that results in a quick deterioration of the nozzle shape (see, e.g., Dumont N., Simonin O., and Habchi C. (2001), Numerical Simulation of Cavitating Flows in Diesel Injectors by a Homogenous Equilibrium Modeling Approach, CAV2001).
Regardless of the above factors appearing to favor conical type nozzles, pure conical nozzles are not always an option in practice because the sharp inlet edges may not last long under high operating pressures of the water flow. However, for “micro-nozzles,” manufacturing an actual “sharp-edge” cone nozzle may not be economically justified in all applications. Therefore, a capillary portion may, in actuality, remain at the inlet due to, for example, high dimensional tolerances in the manufacturing process.
In practice, waterjet instability, and therefore the consequent fluctuations in the waterjet breakup length may arise because of the structural vibration and/or flow pulsation, if the nozzle inlet is sharp (see, e.g., Ramamurthi, K., Patnaik, S. R. (2002), Influence of periodic disturbances on inception of cavitation in sharp-edged orifices, Experiments in Fluids, 33, 720-727). Such disturbances can cause a detached flow to reattach to the nozzle wall and start cavitation. Conventional or otherwise prior art hydroentangling jet strips made of stainless steel tend to undergo severe erosion in a relatively short period of time due to such cavitation. At higher water pressures, the jet strip or nozzles defined thereby will further tend to erode more rapidly. This degradation due to cavitation typically represents a relatively large cost in the process for replacing the jet strips, and also causes an undesirable stoppage in the production line.
Thus, there exists a need for a hydroentangling jet strip device having one or more orifices, wherein orifice erosion and jet strip durability (service life) are improved over existing jet strip configurations.